Zskarn Issues:
No matter the Raid size right now, you still get 4 Embers applied. If you go for example with 10 people (2 Tanks 2 Healers 6 Dps as example) you will have 4 out of 8 People always getting the Ember. Which results into having a 30 sec debuff while Embers get casted around every 15 sec, which can result into multiple person getting one shotted at some because of high stack.
I would recommend either scaling the amount of Embers on the raid size (per 5 person in the raid 1 ember, which would equal the already 4 embers on 20 people) or tune down the damage of that. (while the first option would probably the best at this point in a flex situation)
Possible Magmorax Issues:
Split damage on the soaks depending on the group size and the amount of stacks gained when having a small raid size.